Direct voltage is particularly useful for direct current consumers, which are supplied independently of public energy supply mains. One advantage of the direct voltage is that it can be stored. Thus, for example, direct voltage consumers in vehicles are supplied by a direct voltage, which has in advance been stored in a battery. Direct voltage consumers are, for example, compressors for air conditioning systems or refrigeration units. Often, for security reasons, such vehicles have several direct voltage sources, which can mutually influence each other. Further, with such vehicles, it is often possible to choose from different direct voltage sources, depending on whether the vehicle is in motion or is parked. The use of a direct voltage source may cause increases in voltage, which involve the risk that the connected consumers, electrical components or the direct voltage source itself can be overloaded or even damaged.
Increases in voltage are voltages whose intensity exceeds an anticipated value, for example exceeds a nominal voltage. In this connection, one distinguishes between temporary transient increases in voltage which, for example, occur because of switching operations and increases in voltage which are caused by an incorrect function. In the present case, increases in voltage which are caused by an error are considered more thoroughly. The first step required to avoid such increases in voltage is the detection of the increase in voltage. For this purpose, circuit arrangements are known, with which an increase in voltage can be detected. Increases in voltage are particularly dangerous to semiconductor components.
U.S. Pat. No. 4,713,740 describes a method and a device, in which a direct voltage of a direct voltage source is monitored. An upward or a downward deviation of the direct voltage in relation to a preset value will initiate measures to prevent malfunctions.
Circuits for monitoring the voltage from direct voltage sources are also known from the data sheet of the high voltage resonance controller L 6598 of ST Microelectronics. Here a voltage monitoring by means of diodes and ohmic resistors is performed to protect semiconductor components of the high voltage resonance controller.